1. Field of the Invention
This invention relates to an ion source which makes ions generate in various kinds of substances, and more particularly, relates to a fused metal ion source of the field emission type.
2. Description of the Prior Art
In recent years, the field of technique which makes use of ions of various substances is of very wide range convering, for example, nuclear fusion (heating by plasma, treatment of reactor walls), high energy physics (accelerator), heavy ion science, ion rocket, ion implantation (doping of impurity, surface control of physical properties), film formation (surface coating, developement of film-shaped devices), or others.
With a view to obtaining ions of various substances, a variety of methods are being proposed and put into practice.
By way of example, there is a method of extracting ions by impressing high voltage on substances which are to be ionized, wherein a fused metal ion source of the field emission type is employed as a species of metal ion sources generating ions in various kinds of substances.
FIG. 1 in the accompanying drawings is illustrations showing in brief the cross-sections of examples of conventional fused metal ion sources of the field emission type.
FIG. 1 (a) and (b) each are generally called a needletype, and FIG. 1 (c) is called a capillary-type, as described in, for example, "Studies of field emission gallium ion sources" by A. J. Dixon and A. von Engel, Inst. Phys. Conf. Ser. No. 54 (1980), page 292.
In FIG. 1 (a), the field emission fused metal ion source of the needle-type has a needle, which is made of a material having a high fusing point and the head of which is several to several ten microns in diameter, concentrically built in a nearly cylinder shaped pipe which is made of a different kind of metal also having a high fusing point.
When the needle is heated up, a substance which is to be ionized fuses and infiltrates into the gap between the pipe and the needle to moisten the head of the needle.
Another field emission fused metal ion source of the needle-type shown in FIG. 1 (b) is also made of a material having a high fusing point, but in the form of omitting the nearly cylinder-shaped pipe shown in FIG. 1 (a). The needle, the head of which is several microns, is welded to a heating filament. To the needle clings a substance which is to be ionized. When the needle is heated up, the substance which is to be ionized begins to fuse and comes to moisten the head of the needle.
As for the field emission fused metal ion source of the capillary-type shown in FIG. 1 (c), it also is made of a material of a high fusing point. It constitutes a cone-shaped body being provided with a small diameter throughhole in the longitudinal direction. When this cone-shaped body is heated up, the substance to be ionized fuses, begins to infiltrate into through-hole, and makes the cone-shaped body moist.
When a high voltage is impressed on the metal ion source in such a moist state as mentioned above, then a heavy electric field is present at the head of the ion source, whereby the subtance to be ionized is emitted from the liquid surface, and ionized just before the head of the metal ion source, thus being able to be extracted, at which time the form of the ion emitting face, it is believed, is determined in the ionizing zone. That is, a minute zone of plasma (called a plasma ball) is created just before the head of the metal ion source.
The plasma ball changes according to the working conditions of the field emission fused metal ion source, for example, conditions such as the magnitude of ion current, in proportion to which the beam characteristics such as the angular aperture and the like vary by a large margin.
For all that, there is no way to obtain any control of supplying the substance which is to be ionized to the head zone of conventional field emission fused metal ion sources, so that it is impossible to control an optimum quantity of working flow, and it is difficult to stably operate on the conventional apparatus because of a large quantity of vapor of neutral particles being emitted at the time of ionizing the subtances of high vapor pressure at a temperature near to a fusing point.
Particularly, in the metal ion source of the type shown in FIG. 1 (b), it is hard to operate continuously for long hours on account of the total amount of the substance to be ionized which clings to the needle wet being infinitesimal.
On the other hand, it is desired that the emission of ions from the metal ion source should endure for long hours, while it is required that the angular aperture of ion beam should be stable and small in the case where it is intended to elaborate and control any minute object by the use of ion beam.
However, all conventional field emission fused metal ion sources can not satisfy such requirements as mentioned above.